Recent proposals for high definition television systems ("HDTV") have specified digital transmission techniques. By "digital" is meant that the signal is digitized for passage through a limited bandwidth medium, such as a broadcast channel, or a storage medium, such as tape or disk. It is apparent from the state of the art, that the next generation of television standards will specify that the transmission/storage medium will contain digital, as opposed to analog, modulation. In fact, the present inventor predicted publicly in 1986-1987 that any proposed HDTV scheme not compatible with the widespread existing analog standards (NTSC or PAL, for example) would necessarily employ digital transmission or storage technology.
The drawback with a fully digital television signal format is very simple to state: the bandwidth required for HDTV is very, very high, in excess of approximately 500 megabits per second for a 1050 line by 1500 pixel image array, wherein the picture frame rate is 30 Hz, and each pixel is quantized with eight bits (256 levels).
Since the picture image statistically includes redundant information from frame to frame, existing data compression techniques have been shown to reduce the amount of raw data to a level in the range of 17 to 20 megabits per second, without perceptible degradation of the picture image. Unfortunately, if simple, robust modulation schemes are employed, the bandwidth of a raw data stream of 17-20 megabits per second is well above the typical bandwidth allocated for television transmission (and available for practical recording, particularly in the consumer electronics VCR field).
For example, the regulatory agencies, such as the United States Federal Communications Commission, have decreed that any digital HDTV system must include a digital modulation spectrum capable of fitting into currently available channel spaces left unoccupied under the current NTSC scheme (the so-called "taboo" channels.) For example, in a metropolitan area, if channels 2 and 4 are allocated, channel 3 is left unoccupied as a taboo channel. The digital HDTV signal is to be transmitted as a simulcast with adjacent conventional analog modulated channels and may not interfere with those channels. Thus, any modulation scheme for digital HDTV must stay within the nominal 6 MHz channel bandwidth, and must be transmitted at low power levels and without causing interference to the existing adjacent analog NTSC service channels.
One proposed modulation scheme for digital HDTV is quadrature amplitude modulation. With this scheme, a carrier, often located in the middle of the channel, is modulated in two ways by the data. A first manner of modulation is amplitude: i.e. a choice of amplitude levels is available to code the data. For example, with 16 QAM, two amplitude levels and a sign is available for four different positions. A second manner of modulation is phase modulation: i.e., the carrier changes phase in relation to the data.
FIG. 1 presents a vectorial representation of phase angle and amplitude of the carrier in the case of 16 QAM modulation, wherein 16 different locations are available for the carrier in the phase/amplitude plane shown in FIG. 1. In this example of 16 QAM, the transmission efficiency is about four bits per hertz. Put another way, 24 megabits of information may be carried by modulation contained within a 6 MHz bandwidth of contiguous spectrum. This scheme nominally fits the requirement that the digital HDTV signal fit within the existing TV channel allocation spaces of the taboo channels. Unfortunately, there are some severe drawbacks with this "high density" modulation scheme.
The 16 QAM system is very sensitive to transmission path deficiencies, such as random noise. If a random noise impulse vector NV of a given amplitude is added to a bit vector BV, a resultant at the receiver will cause the bit to be erroneously decoded as another bit. One proposed answer to this error condition is to front end load the digital signal stream with error correction code syndromes which work with error correction circuitry at the receiver to detect, locate and correct burst errors. These schemes are very complicated.
Another, and even more severe problem arises with multi-path (ghosts). The phase shifted ghost signal will cause vector summing at the receiver leading to widespread errors, and general breakdown of the received digital picture.
While these newly proposed systems are called "fully digital", in reality they are subject to the deficiencies of natural analog paths, either transmission or recording, and given the amount of information needed to provide enhanced picture resolution or "high definition", these systems are not very robust in the face of natural channel degradation. In the case of 16 QAM, the digital positions are simply too close together. Advantages achieved in picture resolution are offset by disadvantages of the high density modulation schemes which fare very poorly when passed through condition-degraded analog channels, such as broadcast or recording. In fact, a digital signal may break down completely in the presence of severe multi-path interference, while a conventional analog NTSC signal will result in a coherent, yet low quality picture image on the receiver display.
The state of the art of HDTV is that the effectiveness of a proposed system is measured by comparing the received signal at the display with the signal at the origination point. Such comparisons of signal details in response to a variety of test stimuli may be made with technical and scientific accuracy. Unfortunately, such comparisons shed very little light upon the aesthetic aspects of picture quality, and the practical realities of television usage.
The newly proposed digital HDTV systems typically call for cooperative processes at the encode and decode ends of the path. For example, data compression and data expansion are usually described as symmetrical or complementary processes (i.e. "cooperative"). One example of the prior art is the one proposed by the Motion Picture Expert Group, which proposes using similar circuitries for data compression as are used for data decompression: essentially symmetrical or cooperative processes.
Television displays are typically found in the home. Even the largest television receivers must fit through existing doors found in the home, and thus be not significantly wider than about 30-40 inches. Television display screens are typically viewed at a viewing distance of 8-12 feet in the home. Given these constraints, and given the limitations of the eye-brain perception process (sometimes referred to as the "human visual process"), it is not absolutely necessary to transmit very wide band information. For viewers' satisfaction, only the illusion or the appearance of a high definition television picture image may be provided without a requirement that the degrading medium accommodate high density digital modulation, such as the 16 QAM approach.
Thus, a hitherto unsolved need has been for an improved television system which advantageously combines the best aspects of digital signal format and signal processes in order to improve upon existing channel allocations.